Telco, don’t be confused by the different types of switches. All of those are PCM (time division multiplexed) formats. An external router can connect either a BRI or PRI (ISDN) or T-1 to the switch. The router then provides the path through the cloud (probably T-1, frame relay has too many QOS guarantee problems)to the ISP (www connection).
Attempting to upgrade legacy switches is likely a time consuming and capital consuming exercise. Anyway,in the future they will become scrap iron anyway.
Better (methinks) to add external boxes to gain network interface. Cisco and Lucent (Ascend) both make boxes that will serve the purpose.